Document 2349289

J. Mater. Environ. Sci. 2 (4) (2011) 319-334
ISSN : 2028-2508
El Gouri et al.
A phosphazene compound multipurpose application
-Composite material precursor and reactive flame retardant for epoxy
resin materials
M. El Gouri*, A. El Bachiri, S. E. Hegazi, R. Ziraoui, M. Rafik, A. El Harfi
Laboratory of Macromolecular & Organic Chemistry, Department of Chemistry, Faculty of Sciences, Ibn Tofaïl
University, P.O.Box 133, 14000 Kenitra, Morocco
Received in 28 Feb 2011, Revised 18 July 2011, Accepted 18 July 2011.
* Corresponding author. E-mail address: [email protected] .Tel.: +212 537329400; fax: +212 537329433.
Abstract
Cyclotriphosphazene containing the epoxy group hexaglycidyl cyclotriphosphazene (HGCP) was
synthesized in one step. The reaction was a nucleophilic substitution of cyclophosphazene chlorine by the
epoxy function of 2,3-Epoxy-1-propanol in presence of triethylamine. The purified compound of the
reaction was characterized by FTIR, 31P, 1H, and 13C -NMR nuclear magnetic resonance spectroscopy. This
reactive monomer that is inherently flamed retarding contain P and N, can be used on their own as
composite material precursor or added to current bulk commercial polymer diglycidylether of bisphenol A
(DGEBA) to enhance flame retardancy. The thermal stability and flame retardancy of HGCP thermoset with
MDA curing agent and its blend as flame retardant with DGEBA were checked by thermal gravimetric
analysis coupled with infrared spectoscopy and the UL-94 vertical test. The correlation between these
properties (thermal stability and flame retardancy) and the HGCP contents (phosphorus content) were
discussed. The results show that HGCP as composite material precursor lead a good material with thermal
stability at elevated temperature compared with DGEBA thermoset with MDA curing agent.
HGCP as flame retardant presents a good dispersion in DGEBA, and the blend thermoset with 4,4’methylene-dianiline (MDA) curing agent leads to a significant improvement of the thermal stability at
elevated temperature with higher char yields compared with pure DGEBA thermoset with the same curing
agent. HGCP acts with an intumescent char-forming and gas action by CO2 gas emission which its act to
dilute the combustible gas. Improvement has also been observed in the fire behaviour of blend.
Keywords: Synthesis; epoxy resin; flame retardant; thermal stability.
grade is required, but the fire risk is a major
drawback of these materials [1].
There are two approaches to achieve flame
retardancy for polymers generally known as the
“addition” and the “reaction”, and the latter is
1. Introduction
Epoxy resins are widely applied as advanced
composite matrices in electronic/electrical
industries where a remarkable flame-retardant
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El Gouri et al.
given much attention to recently [2].
Traditionally, brominated reactive compounds [3]
are used as co-monomers with epoxy resins to
obtain fire-retardant materials. However, flameretardant epoxy resins containing bromine can
produce corrosive and obscuring smoke and may
give super-toxic halogenated dibenzodixines and
dibazofurans with deleterious effects on the
environment and human health. Recently, in
consideration of environmental problems,
researches for halogen-free fire-retardant epoxy
resins have received a great deal of attention
[4,5]. Phosphorylation is considered to be one of
the most efficient methods to confer flame
retardancy on epoxy resins [6-8], whereas
phosphate-based epoxy resins could possess
excellent flame retardancy only when using
amine curing agents. Some studies indicated that
the flame-retardant efficiency significantly
improved when phosphorus and nitrogen existed
simultaneously in the curing system of epoxy
resin. Therefore, the phosphorus-nitrogen
synergistic effect on flame retardancy is very
interesting [9].
In recent years, there has been considerable
interest in the phosphazene-based family of
materials because they not only have a wide range
of thermal and chemical stabilities, but also can
provide improved thermal and flame-retardant
properties to polymers and their composites [1015]. Hexachlorocyclotriphosphazene is a versatile
starting oligomer for the synthesis of
phosphazene-based polymers. The chlorine
groups attached to the phosphorus atoms are
easily substituted by various nucleophiles to form
reactive cyclotriphosphazenes.
Cyclotriphosphazene, as a ring compound
consisting of alternating phosphorus and nitrogen
atoms with two substituents attached to the
phosphorus atoms, exhibits unusual thermal
properties such as flame retardancy and selfextinguishability [16,17]. Cyclotriphosphazene
has several advantages as a reactive flameretardant functional oligomer. First, the flexible
synthetic methodology can be developed for
preparation
of
cyclotriphosphazene-based
copolymer with various substituents, which
allows us to obtain multifunctional initiators or
terminators with ease. Second, thermal and
nonflammable
properties
of
the
cyclotriphosphazene moieties can be conferred to
the resulting polymers, especially, of low
molecular weights [12,13,18-20].
Therefore, when cyclotriphosphazenes are
incorporated into the network of thermoset
polymers, they can increase the thermal property
and flame retardancy of the polymers because of
phosphorous and nitrogen flame-retardant
synergy. The reason is that the thermal
decomposition of the phosphazene-based
polymers is an endothermic process, and
phosphate,
metaphosphate,
polyphosphate
generated in the thermal decomposition form a
nonvolatile protective film on the surface of the
polymer to isolate it from the air; meanwhile, the
inflammable gases released such as CO2, NH3 and
N2 cut off the supply of oxygen so as to achieve
the aims of synergistic flame retardancy [21-24].
The phosphazene-based polymers have more
effective flame retardancy than any other flameretardants, making them a new focus [25-29].
However, the phosphazene-based polymers used
as a flame-retardant component with epoxy resins
are seldom reported [28].
In this report, we developed a novel
nonflammable halogen-free epoxy resin by
incorporating the cyclotriphosphazene group, and
investigated the thermal property and flameretardancy of this thermoset resin with MDA
curing agent.
The synthesized phosphazene-containing
epoxy group (HGCP) is a phosphazene compound
multipurpose, it can be used both as composite
material precursor and as flame retardant in
epoxy resins.
2. Experimental
2.1. Materials
Hexachlorocyclotriphosphazene, 4,4’-methylenedianiline (MDA) (Aldrich chemical company)
and 2,3-epoxy-1-propanol (ACROS chemical
company, 99%) stored at temperature of 4°C to 6
°C, DGEBA (Epon828). All these materials were
used without any further purification.
2.2. Instrumentation
Infrared spectra were recorded on a Vertex 70
FT-IR spectrophotometer using KBr technique.
1
H, 13C, and 31P nuclear magnetic resonance
(NMR) Spectra were obtained on a Bruker
AVANCE 300 NMR Spectrometer using CDCl3
as solvent. Chemical shifts (δ-scale) are quoted in
parts per million and following abbreviations are
used: s = singlet; m = multiplet.
Thermogravimetric analysis (TGA and
DTA) were carried out on an SETARAM
thermogravimetric analyzer (The SETSYS
evolution) with a heating rate of 10°C/min from
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El Gouri et al.
room temperature to 1000 °C under air
atmosphere.
The DSC measurements were carried out
using a PYRIS6 DSC in nitrogen atmosphere,
with a heating rate of 10°C/min under N2 from 40
to 300°C. UL 94 standard test carried out in terms
of the method proposed by Underwriter
laboratory. It was used to evaluate the fire
retardancy properties of the materials.
The microstructures of samples and the
chars
were
recorded
using
“MEB
ENVIRONNEMENTAL” scanning electron
microscope (SEM) coupled with (EDX)
elementary analysis.
.
2.3. Synthesis of hexaglycidyl
cyclotriphosphazene (HGCP)
Hexaglycidyl cyclotriphosphazene (HGCP) was
synthesized in one step, according to the
procedure literature [30,33]. The reaction was a
nucleophilic substitution of cyclophosphazene
chlorine by the epoxy function of 2,3-epoxy-1propanol in presence of triethylamine (Figure 1).
The purified compound of the reaction was
characterized by FTIR, 31P, 1H, and 13C -NMR
nuclear magnetic resonance spectroscopy.
O
H2 C
O
CH
CH
CH2
H2C
H2
C
CH
O
Cl
Cl
P
P Cl
N
N
O
Cl
P O
HO CH2 CH
CH2
C
H2
N
CH
CH2
O
P
NEt3
+
O
N
N
P
Cl
CH2
O P
Cl
N
CH2
O
Toluene
H2C
O
O
CH
O
+
6[Cl-,+NHEt3]
H2 C
CH
O
C
H2
C
H2
Figure 1. Synthesis of hexaglycidyl Cyclotriphosphazene (HGCP)
2.4. Sample preparation
Epoxies (HGCP, DGEBA and there blend) were
warmed to melt and the curing agent (Scheme 1)
added and mixed until homogeneous. The resinhardener mixture was then poured into preheated
molds and cured in a forced convection oven to
make samples (Figure 2).
The mixture of the epoxy resin with 4,4’methylene-dianiline (MDA) curing agent before
the crosslinking is carried out according to the
protocol adopted by Levan [34]. The formulations
and cure schedules were as follows:
*The samples were prepared by mixing
stoichiometric amount of MDA and epoxy resins
HGCP, DGEBA and there blend.
*The samples thus prepared were
underwent the cycle of heating: one night with
70°C, three hours with l00°C, two hours with
120°C, one hour with 140°C and 30 minutes with
150°C.
Figure 2. Technical of samples preparation
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Table 1. Formulations prepared of (DGEBA/HGCP) thermosets with MDA curing agent
Samples
%DGEBA per
100g of blend
% HGCP per
100g of blend
1
2
3
4
5
100
95
90
85
80
0
5
10
15
20
Parts of MDA to thermoset
100g of (DGEBA/HGCP)
blend (g)
29
29
30
32
33
groups), it was expected to act as crosslinking
agent (Scheme 1).
3. Results and discussions
3.1. Composite material precursor application
HGCP have been examined for its application as
composite material precursor [33]. Compared to
DGEBA, the thermal stability and flame
retardancy of HGCP thermoset with MDA curing
agent were checked by thermal gravimetric
analysis coupled with infrared spectoscopy and
the UL-94 vertical test. The correlation between
these properties (thermal stability and flame
retardancy) were discussed.
H2N
CH2
Scheme 1. Chemical structures of 4,4’-methylenedianiline (MDA) using as curing epoxy resins.
In order to evaluate the effect of structure
of HGCP resin compared to that of DGEBA on
curing behavior with MDA, studies were carried
out using stoichiometric ratio of (HGCP and
MDA), (DGEBA and MDA). The characteristic
curing temperatures are summarized in table II.
Therefore, onset temperature T0 of
exotherm may be used as a criterion for
evaluating the relative reactivity of various resins.
3.1.1. Reactivity of HGCP and DGEBA toward
the MDA Curing Agent
MDA is a reactive agent (the free NH2
groups make it capable to react with epoxy
100
1 0 0
87
80
8 0
77
0
T : Onset temperature of exotherm (°C)
NH2
H
G
C
P
D
E
R
3 3 1
E p o x y
E
p o n 8 2 8
D
E
R
7 3 2
r e s i n
Figure 3. The Initial Curing Temperature of HGCP and DGEBA Epoxy resins cured by the MDA curing
agent.
The exothermic peaks were observed in the
system (HGCP/MDA): the onset temperature of
exothermic peak was observed at 77.13 °C, while
the systems (DGEBA/MDA) such as (DER
331/MDA),
(Epon828/MDA)
and
(DER732/MDA) according to other studies [28]
the onset temperature of exothermic peak was
higher than that observed in (HGCP/MDA)
system (Figure 3).
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We can conclude that the HGCP is more reactive
toward MDA than the DGEBA epoxy resin,
because the amine–epoxy cross-linking reactions
of (HGCP/MDA) was faster than that of
(DGEBA/MDA). This may be ascribed to the
electronic effect. The polymerization of an epoxy
resin with an amine is considered to occur via a
nucleophilic attack of the amine nitrogen on the
methylene carbon atom of the epoxy group. The
electron withdrawing cyclotriphosphazene ring in
the HGCP reduces the electron density of the
methylene carbon atom and makes the reaction
between the oxirane ring and the amine easier.
On the other hand, in a densely cross-linked
structure, the HGCP hydroxyls and residual
epoxy groups were likely to remain immobilized
and unable to react together easily, lead the end of
the exotherm until 231 °C.
that the thermal stability of the HGCP/MDA
polymer was less than that of the Epon828/MDA
polymer. This is attributed to the less stable P-OC bond linkage. This result is the same with that
reported in the previous study [35].
This degradation as we can see in DTA
curve (figure 6) has an endothermic effect.
Nevertheless, the percentage of the weight loss of
HGCP/MDA before reaching 257 °C was less
than 5%. This weight loss could be ascribed to the
decomposition of the phosphate groups that
caused the formation of the phosphorus-rich
residues to inhibit further decomposition of the
polymer. Consequently, the second maximum
weight-loss temperature (Tmax2) of the
HGCP/MDA polymer was higher than the Tmax of
the Epon828/MDA polymer (as listed in table 2),
implying that the thermal stability of the resin at a
high temperature was improved as the phosphate
and the cyclotriphosphazene moieties were
covalently incorporated into the epoxy resins.
This phenomenon, in agreement with other
studies [36], played an important role in
improving the flame-retardant properties of the
resins. The second degradation as we can see in
DTA curve (figure 6) is exothermic which might
be ascribed to the generation of P-O-P. The
appearance of P-O-P group can be considered as a
crosslinker linking to different species, resulting
in the formation of complex phosphorus
structures [37].
3.1.2. Thermal behaviour and mechanism
degradation
The thermal properties of the cured epoxy
polymers were evaluated by TGA under air.
Figure 4 shows the TGA thermograms of the
cured
HGCP/MDA
and
Epon828/MDA
polymers. The Epon828/MDA polymer exhibited
a 5% weight loss at 311 °C and a rapid weight
loss around 362 °C. Unlike the one-stage weightloss behaviour of the Epon828/MDA polymer,
the HGCP/MDA polymer showed two stages
weight loss (figure 5). The first maximum weightloss temperature (Tmax1) around 258 °C indicates
.
Figure 4. Thermogravimetric analysis of samples (HGCP/MDA) and (DGEBA/MDA) in air atmosphere at
10°C min-1 heating rate
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Figure 5. Thermogravimetric analysis of sample (HGCP/MDA) and first derivative under air atmosphere
and with 10°C min-1 heating rate.
Figure 6. DTA analysis of sample (HGCP/MDA) in air atmosphere at a 10°C min-1
Table 2. Thermogravimetric data of the HGCP.
Samples
Temperatures of weight loss
from
TGA (°C) (in Air)
Tmax Tmax
5% 10%
1
2
Char (%)
at
500°C
HGCP/MDA
257
263
258
512
71.9
Epon 828
/MDA
311
325
-
362
20
324
HeatFlow DTA
Tmax1: 265.9°C
Endothermic
effect
Enthalpy
38.7 µV.s/mg
Tmax2:491.7°C
Exothermic
effect
Enthalpy
-51.4 µV.s/mg
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barrier to inhibit gaseous products from diffusing
to the flame and to shield the polymer surface
from heat and air. This act is an agreement with
the result of study of gases evolved during TGA
trials in air atmosphere by means of
thermogravimetry coupled with Fourier transform
infrared spectroscopy (TG–FTIR) (figure 9),
which indicate the reduction of gaseous products
in the case of HGCP/MDA compared with
DGEBA/MDA sample. But we can observe the
CO2 emission (2250 cm-1) which persist in gas
phase’s; such a gas can plays an important role in
autoextinguiblity of sample.
Figure 7 shows the FTIR spectra of
residual products of HGCP/MDA degraded at
different temperatures. The absorption of
cyclophosphazene (1183 cm_1 and 1249 cm_1) is
still rather strong at 250 °C. However, in the
FTIR spectra of HGCP/MDA , the absorption
peak at 1013 cm-1 due to P-O-C bond decrease
with the increasing temperature and disappear at
500 °C. The characteristic absorption peaks for
P=N at about 1209 cm_1 and for ester group at
1183 cm_1 also disappear at 500 °C.
Moreover, a new peak at 1079 cm_1, which
might be ascribed to the generation of
P-O-P,
and appear at 500 °C. This is in agreement also
with the TGA, DTG and DTA data for our
sample (HGCP/MDA).
The formation of phosphorus-rich char in
the decomposition of cyclotriphosphazenecontaining epoxy resins not only increased the
weight-loss temperature in the high-temperature
region but also resulted in a high char yield
(figure 8).
This category of flame retarding
mechanism is that known as ‘intumescent’, in
which materials swell when exposed to fire or
heat to form a porous foamed mass, usually
carbonaceous, which acts in the condensed phase
promoting char formation on the surface as a
In general, phosphazenes give high char yields at
elevate temperatures and also undergo a high
degree of cross-linking to form a dense
ultrastructure [38-41].
An increase in char formation limit the
production of combustible carbon-containing
gases, decrease the exothermicity due to pyrolysis
reactions, and decrease the thermal conductivity
of the surface of a burning material. Therefore,
the flammability of the material is reduced. UL94 test results are also given in table 3. V-0 rating
was obtained for HGCP/MDA. By way of
comparison with DGEBA, HGCP exhibited selfextinguishing characteristics in the UL94 V test.
Figure 7. FTIR spectra the thermal degradation of HGCP/MDA sample in room temperature, 250 and 500
°C.
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Figure 8. Evolution of samples volume of (HGCP /MDA) and (DGEBA / MDA) at different temperature
heating in the range of RT—500 °C.
(b)
(a)
Figure 9. FT-IR spectrum of gas evolved during TGA of DGEBA/MDA (a) and HGCP/MDA (b)
Table 3. Flammability data
Epoxy resin/MDA
Dripping
UL94 rating (*)
Remarks
No
V0
Very light smoke
HGCP/MDA
No
Burning
Very strong black smoke
DGEBA/MDA
(*)
V-0 (vertical burn classification): Burning stops within 10 seconds. No flaming drips are allowed.
Flaming drips, widely recognized as a main source for the spread of flames, distinguish V1 from V2.
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3.2. Reactive flame retardant application
HGCP have been examined for its application as
flame retardant for composite material based on
DGEBA epoxy resin [30]. The thermal stability
and flame retardancy of DGEBA flame retarded
with different amount of HGCP were thermoset
with MDA curing agent and were checked by
thermal gravimetric analysis coupled with
infrared spectoscopy and the UL-94 vertical test.
The correlation between these properties (thermal
stability and flame retardancy) were discussed.
flame retardancy properties. Since the HGCP is
coated with organic groups such as epoxy groups
and DGEBA is an organic prepolymer epoxy
resin, well dispersion was expected. Due to
compatibility occurring between two species,
flame retardant additive disperse in prepolymer
DGEBA and no agglomerates are observed
(Figure 10). In the opposite, this result compared
with the mixture of DGEBA and [NPCl2]3 we can
observe the agglomeration of this last which
indicates the no compatibility of this inorganic
compound in the organic matrix of DGEBA. This
is an argument for our compound HGCP of its
well dispersion in DGEBA due to the organic
groups of epoxy functions attached in
cyclophosphazene
3.2.1. Dispersion of flame retardant with base
polymer matrix
- Physical mixing of DGEBA and HGCP
It is important for the flame retardant additive to
be dispersed in polymer matrix for improved
Figure 10. The SEM results of the physically blended DGEBA and flame retardant “HGCP” compound in
melt blender, DGEBA/[NPCl2]3 blends. [30]
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3.2.2. Reactivity of (DGEBA/HGCP) blend
toward the MDA curing agent
MDA is a reactive agent (the free NH2 groups
make it capable to react with epoxy groups), it
was expected to act as crosslinking agent.
In order to evaluate the effect of
(HGCP/DGEBA) blend on curing behavior with
MDA, studies were carried out using a DSC
stoichiometric ratio of (DGEBA/20%HGCP) and
MDA, with a heating rate of 10 °C/min under
nitrogen atmosphere from 40 to 300 °C.
The characteristic curing temperatures are
summarized in table 4.
TGA as illustrated in below figures. Char yields
were investigated to obtain information the fire
resistance of samples. Figure 11 shows the TGA
and DTG curves for the DGEBA, HGCP and
there blends cured by MDA. The pure DGEBA
sample began to lose its weight at about 300 °C
and degraded in the range of 300–450 °C with
around 50% weight loss and little residue
remained at above 600 °C.
The HGCP/DGEBA blends have a lower
initial decomposition temperature of around 247
°C, and they have the same degradation behaviour
in the range of 247–342 °C. This is attributed to
the less stable P-O-C bond linkage and to the easy
decomposition of organic network and the highest
phosphorus content in HGCP molecule. This
result is the same with that reported in the
previous study [42]. For DGEBA sample, the
weight loss at 370 °C reached to 60% due to its
more volatile side organic group. The char yield
of
(DGEBA/HGCP/MDA)
at
elevated
temperature increase with the increasing of
HGCP amount in sample because the phosphorus
content increase gradually. So, the TGA curves
for (DGEBA/MDA) and (DGEBA/HGCP/MDA)
samples show significant difference over 600 °C.
The (DGEBA/HGCP/MDA) samples present a
slower mass loss rate and a highest char yield due
to the phosphorus content of HGCP and the
organic content in DGEBA.
DGEBA without flame retardant (HGCP)
begins to decompose first at 300°C, and second
phase of weight loss was at 486°C and totally
degraded at 650°C. When HGCP is added to
DGEBA matrix, thermal stability of compound
increases as expected. From the first derivative it
is observed that existence of HGCP decreases
amount of exothermic reactions. Addition of
HGCP
pulled
first
decomposition
of
(DGEBA/HGCP/MDA) blends around 245°C.
However, the stability increased up to 800°C.
DGEBA with HGCP preserves 20% of its
original weight at 800°C in contrast with neat
DGEBA compound was totally disappeared.
Table 4. The Initial Curing Temperature of
HGCP and DGEBA Epoxy resins cured by the
MDA curing agent. [30]
T0: Onset
Epoxy Resin/ MDA
temperature of
exotherm (°C)
HGCP
77
DGEBA
95
DGEBA/20%HGCP
93
Therefore,
the
exothermic
onset
temperature (T0) may be used as a criterion for
evaluating the relative reactivity of resins.
In the case of (HGCP/MDA) system, the
onset temperature of exothermic peak was
observed at 77 °C, while the (DGEBA/
20%HGCP/MDA) and (DGEBA/MDA) systems,
the onset temperature of exothermic peak was
higher than that observed in (HGCP/MDA)
system, around the 95°C (Table 4). But the
addition of HGCP in DGEBA gives a large
exothermic peak and no significant effect on the
onset temperature of DGEBA epoxy resin. And it
evident that all blends witch HGCP is less than
20% can gives no effect on the onset temperature
of DGEBA epoxy resin.
We can conclude also that the HGCP is
more reactive toward MDA when it is only mixed
with the curing agent because the epoxy groups of
HGCP are in direct contact with amine of MDA
and it makes the amine–epoxy cross-linking
reactions of (HGCP/MDA) faster than that of
blend (DGEBA/20%HGCP/MDA).
3.2.4. Degradation mechanism
- Condensed phase action
The TGA results indicated that cyclophosphazenes play important roles to increase the
thermal stability of DGEBA at elevated
temperatures. However, its stability at lower
temperatures decreases. To further investigate
their influence, in-situ FTIR was used to monitor
the thermal degradation of DGEBA, HGCP and
the blend from room temperature (RT) to 500 °C.
3.2.3. Thermal behavior
The thermal stability of a polymeric material is
very important while used as a flame retardant
which mainly concerns the release of
decomposition products and the formation of a
char. Thermal stability of HGCP an its blend with
DGEBA cured by MDA were characterized using
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.
Figure 11. Thermogravimetric analysis (A) and first derivative (B) of samples (HGCP/MDA),
(DGEBA/MDA) and (DGEBA/HGCP/MDA) under nitrogen atmosphere at a 10°C min-1 heating rate.
Figure 12. FTIR spectra the thermal degradation of DGEBA/MDA sample in room temperature to 500 °C.
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200 °C
250°C
300°C
P-O-C
P=N
Charring of sample
500°C
P-O-P
Figure 13. FTIR spectra the thermal degradation of DGEBA/HGCP/MDA sample in room
temperature to 500 °C.
The residual products of (DGEBA/MDA)
sample degraded at different temperatures were
analysed by FTIR (Figure 12). There was little
change in the absorption peaks of the FTIR
spectra before 300 °C, which is consistence with
the TGA result. The absorption of ether group
(1089 cm_1 and 1012 cm_1) is still rather strong at
300 °C. No new absorption bands have been
found during the thermal degradation period.
However,
in
the
FTIR
spectra
of
(DGEBA/20%HGCP/MDA) sample (Figure 13),
the absorption peaks at 1013 cm_1 and 903 cm_1
due to P-O-C bond decrease with the increasing
temperature and disappear at 250 °C. The
characteristic absorption peaks for P=N at about
1240 cm_1 and for ether group at 1170 cm_1, 1240
cm_1 also disappear at elevated temperatures.
Compared with those in (DGEBA/MDA) sample,
the absorption peaks at 2850–2980 cm_1 (C–H) in
the spectra of (DGEBA/20%HGCP/MDA)
sample decrease more quickly with the increasing
temperature.
Moreover, a few new peaks at 1079 cm_1,
which might be ascribed to the generation of
P-O-P, and appear at 500 °C. This is in agreement
also with the TGA and DTG data for our samples.
Similar to the mechanism which has been
reported [43,44], one possible reason for the
result is that cyclophoshazenes could experience
the reactions as follows while heating (Figure
14):
Figure 14. Mechanism decomposition of HGCP
in DGEBA matrix.
The obtained structure could act as an acid
catalyst, which would accelerate the cleavage of
side groups and the breaking of ether groups (PO-C) in HGCP and its blend with DGEBA. Then,
the degradation products reacted with the residue
of (DGEBA/20%HGCP/MDA) and formed more
stable structures. The appearance of P-O-P group
is considered a crosslinker linking to different
species, resulting in the formation of complex
phosphorus structures [42]. This is why the
thermal degradation of the blend containing
cyclophosphazenes is much slower than DGEBA
at above 400 °C (Figure 11).
The formation of phosphorus-rich char
(Figure 15) in the decomposition of blend was
observed with high char in high-temperature
region.
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El Gouri et al.
We can conclude that HGCP has an
intumescent action in condensed phase of blend.
combustion
surface.
These
mentioned
inflammable gases can cut off the supply of
oxygen.
This act is an agreement with the results
of the study of gases evolved during TGA trials in
air atmosphere by means of thermogravimetry
coupled with Fourier transform infrared
spectroscopy (TG–FTIR) (Figure 16) [45].
- Gas phase action
The cyclotriphosphazene moieties can
release the inflammable gases such as CO2, NH3
and N2 during burning to dilute the hot
atmosphere and cool the pyrolysis zone at the
Figure 15. The SEM results and EDX analysis of the barrier formed on the surface of
(DGEBA/HGCP/MDA) sample at 600°C.
22/01/2009
22/01/2009
(b)
(a)
CO2
CO2
C:\SPECTRES\2009\TGA\SR4 b.0
SR4 b
Couplage ATG-IR
C:\SPECTRES\2009\TGA\sr7.1
sr7
Couplage ATG-IR
Figure 16. IR of Gases involved during the TGA analysis of DGEBA/MDA sample: (a) and
DGEBA/HGCP/MDA sample(b)
The incorporation of HGCP to DGEBA
epoxy resin decreases the time and the
temperature of CO2 emission, compared to the
pure (DGEBA/MDA) sample. This act can play a
very important role to the auto extinguibility of
the sample.
We can also observe the H2O emission,
which can be due to the condensation between PO-H, resulting from the degradation of HGCP.
This can be in agreement with the mechanism
giving in figure 14.
3.2.5. SEM-EDX and TGA-IR analysis of flame
retarded DGEBA with HGCP
SEM pictures of chars of DGEBA/HGCP/MDA
are shown in Figure 17. We can see from the
pictures that there are formation of bubbles at
interior surface starting the fist degradation in the
range of 247–342 °C. From this period, HGCP
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J. Mater. Environ. Sci. 2 (4) (2011) 319-334
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El Gouri et al.
began to release non-flammable gases such as
CO2, NH3 and N2 [45].
These mentioned inflammable gases inside
bubbles can create gullys on the surface of sample
which are considered channels to dilute the hot
atmosphere and cool the pyrolysis zone at the
combustion surface.
These mentioned inflammable gases can
cut off the supply of oxygen. This act is an
agreement with the results of the study of gases
evolved during TGA trials in air atmosphere by
means of thermogravimetry coupled with Fourier
transform infrared spectroscopy (TG-FTIR),
which indicate the continues emission of nonflammable gases such as CO2 during thermal
degradation of sample (Figure 16).
The pores formed during charring are
evident in Figure 3. However, HGCP is also able
to limit the char porosity, forming extensive
glass-like surfaces and closing some pores at
higher temperatures (Figure 17), and so limiting
the access of oxygen to the undegraded polymer
and the expulsion of pyrolysis gases. These
results indicate that when char formation was
promoted, the flame retardancy of cured DGEBA
epoxy resin with HGCP linked in the network
was significantly increased.
This kind of structure is helpful for heat
insulation and hindering of mass transfer. Thus,
this kind of char can effectively lower the
temperature of polymer substrate under it and
hinder gas exchange between upperlayer and
downlayer of it, and as a consequence improve
the flame retardancy of the material.
The phosphorus contents of blends at
various temperatures were measured with EDX
analysis, and are listed in Figure 17. While
heating the blends from room temperature to 500
°C, the phosphorus contents in blends increased
steadily.
Figure 17. SEM images of inflammable gas evolution and char formation process for intumescent
protection of burned specimen of (DGEBA/HGCP/MDA) and its EDX analysis [45].
The results indicate that phosphorus
element could remain exclusively in the residue
during the thermal degradation. Combined with
the results from FTIR analysis, the formation of
new structures with rich phosphorus in the blends
is crucial, which are more stable and act as a
protective layer at elevated temperature.
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3.4. Flame-retardant properties
Flame-retardant
properties
of
the
(DGEBA/HGCP) blends thermoset with MDA
curing agent were further evaluated by UL94
vertical flammability test, and the results were
listed in table 5. It is clear that the incorporation
of HGCP in DGEBA thermosets with MDA
curing agent gains excellent flame retardancy in
comparison with the flammable DGEBA curing
system reported in lots of literatures [46]. The
samples successfully achieved the flammability
rating of UL94. It’s reached the V-0 class in
UL94 vertical test.
By way of comparison with DGEBA, the
incorporation of HGCP to DGEBA exhibited selfextinguishing characteristics in the UL94 vertical
test.
The formation of the important char during
the flammability test of (DGEBA/HGCP/MDA)
blends compared with the (DGEBA/MDA)
sample, improves and is in agreement also with
the previous results. In addition, a loading of 5%,
a V-0 grade of UL94 can be reached.
In view of flame retardant efficiency and
economic, the optimum addition amount of
HGCP is from 5%.
Table 5. Flammability data
Formulations
Curing agent Dripping
UL94 rating (*)
Remarks
DGEBA/0%HGCP
MDA
No
V1
Very strong black smoke
DGEBA/ 5%HGCP
MDA
No
V0
Light smoke
DGEBA/10%HGCP
MDA
No
V0
Light smoke
DGEBA/15%HGCP
MDA
No
V0
Light smoke
DGEBA/20%HGCP
MDA
No
V0
Light smoke
(*)
V-0 (vertical burn classification): Burning stops within 10 seconds. No flaming drips are allowed.
Flaming drips, widely recognized as a main source for the spread of flames, distinguish V1 from V2.
evidence of significant flame retardancy. It was
believed to result from the production of a
protective char layer which inhibited the overall
combustion of the blend. No obviously improved
flammability was found in the blend containing
HGCP.
The result is attributed to the condensed and
gas phase mechanism and homogeneous
distribution of HGCP in DGEBA matrix.
The halogen-free flame retardant synthesized
in this study using for epoxy resin, has potential
applications in electric and electronic fields in
consideration of the environment and human health.
4. Conclusion
This research constitutes the report of the
multipurpose application of cyclotriphosphazenecontaining epoxy group (HGCP). It have been
examined for their application as composite
material precursor and reactive flame retardant.
The comparison of the thermal behaviours of the
epoxy resins revealed an improvement of thermal
inertia of the HGCP compared to the DGEBA
(Epon828), which is mainly due to the difference
in their chemical structures incarnated by the
presence of phosphorus and by the more compact
structure in the case of the HGCP.
The observed characteristics of the
polymer, namely good thermal stability, solubility
in practical solvents, make this polymer a suitable
candidate for the development of flame-retardant
materials, composite systems and advanced
materials.
In the second application of HGCP as a
reactive flame retardant, the addition of HGCP
can effectively improve the thermal stability of
DGEBA blend at elevated temperatures.
However, the initial decomposition temperature
of the blend decreased due to the breakage of PO-C bonding.
The presence of HGCP in the blend had
positive effect on overall flammability. At the
levels of 20% HGCP in the blend, there was
Acknowledgements
This research was supported by the grant of
ELCOM under grant of PROTARS III n° D 13/11
rubric n° II-50-65.
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